Secondary batteries such as lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide variety of applications. Consequently, in recent years, studies have been made to improve battery members such as electrodes for the purpose of achieving even higher secondary battery performance.
An electrode for a secondary battery, such as a lithium ion secondary battery, generally includes a current collector and an electrode mixed material layer formed on the current collector. The electrode mixed material layer is formed, for example, by applying, onto the current collector, a slurry composition in which an electrode active material, a binder composition containing a binding material, and so forth are dispersed in a dispersion medium, and drying the applied slurry composition.
In recent years, there have been attempts to improve binder compositions used in the formation of electrode mixed material layers in order to further improve secondary battery performance. In one specific example, it has been proposed that binding capacity between particles of an electrode active material, or between the electrode active material and a current collector, can be increased and secondary battery performance can be improved through use of a binder composition that contains two types of particulate polymers of differing particle diameters as a binding material.
More specifically, PTL 1, for example, proposes a technique that increases binding capacity between particles of an electrode active material, or between the electrode active material and a current collector, and that improves secondary battery cycle characteristics through use of a binding material including first rubbery resin particles composed of styrene-butadiene rubber particles having an average particle diameter of at least 130 nm and second rubbery resin particles composed of nitrile rubber particles having an average particle diameter of less than 130 nm.
In another example, PTL 2 proposes a technique that improves pressing processability of an electrode for secondary battery-use while increasing binding capacity between an electrode active material and a current collector through use of a binding material including a polymer latex (a) having a number average particle diameter of from 80 nm to 120 nm, a glass transition temperature of from 5° C. to 50° C., and a toluene gel content of at least 70%, and a polymer latex (b) having a number average particle diameter of from 150 nm to 280 nm, a glass transition temperature of from −50° C. to 0° C., and a toluene gel content of at least 70%.